Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Minor mdoc improvements. Also remove unnecessary csup reference.

Fix some Xr references:

- ada(4): ad(4) - removed, ada(4) would be a self-referencing entry
- cd(4): ad(4) -> ada(4)
- da(4): ad(4) -> ada(4)
- DEVICE_PROBE(9): ugen(5) -> ugen(4)
- ed(4): dhclinet(8) -> dhclient(8) (typo)
- lmc(4): Netgraph(4) -> netgraph(4)
- security(7): rc.conf(8) -> rc.conf(5)
- sfxge(4): cpuset(8) -> cpuset(1)
- sbp(4): sysctl(1) -> sysctl(8)
- portindex(5): build(1) -> build(7)
- u3g(4): usbconfig(5) -> usbconfig(8)
- usb_quirk(4): usbconfig(5) -> usbconfig(8)

Found with: textproc/igor
MFC after: 3 days

- Re-order SEE ALSO entries

Noticed by: brueffer
Approved by: trhodes

- Add initial version of portindex(5) manual page

PR: docs/70652
Submitted by: Paul Armstrong <psa otoh.org>
Reviewed by: pav keramida trhodes
Approved by: keramida
MFC after: 3 days